The present invention relates to a method for producing graded-index profiles, i.e., graded refractive-index profiles, in polymeric optical fibers.
The use of optical waveguides made of quartz-glass fibers for optical signal transmission is common practice today in the field of communications engineering. Apart from the material specifications and numerical aperture, the transmission characteristics and transmission quality are essentially determined by the usually radially symmetrical refractive-index profile in the fiber core. Typical transmission characteristics are fiber attenuation, material and modal dispersion, transmission rate and bandwidth/length product (as presented, for example, in: John M. Senior: Optical Fiber Communications, Second Edition; Prentice Hall international series in optoelectronics, 1992).
At present, besides the xe2x80x9cmultimode graded-index fibersxe2x80x9d, it is predominantly single-mode step-index fibers that are used in practice for telecommunications. Standard multimode graded-index fibers have an axially symmetrical, parabolic refractive-index profile The diameter of the fiber core is normally 50 xcexcm. Conversely, standard single-mode step-index fibers have a stepped refractive-index profile in the core region, with a mode-field diameter in the core of normally between 9 xcexcm and 10 xcexcm. The normally used total diameter of both types of fiber is 125 xcexcm.
A decisive factor for the satisfactory transmission characteristics of these fibers is the sharp decrease in modal dispersion owing to the selected refractive-index profile, this considerably reducing the signal overlap which is detectable in the case of long transmission distances. In a single-mode fiber, ideally only one mode is capable of propagation and consequently possesses excellent transmission characteristics.
In addition to fibers based on quartz glass, use is also made of polymeric optical waveguides for optical signal transmission (for information on the level of development, see: O. Zieman: Grundlagen und Anwendungen optischer Polymerfasern [Bases and Applications of Optical Polymer Fibers], Der Femmelde-ingenieur, 50, No. 11/12, 1996). Besides fibers made of polycarbonate, polystyrene and polyvinyl chloride, preference is presently overwhelmingly given to fibers made of polymethyl methacrylate (hereinafter abbreviated to PMMA). The advantages of PMMA fibers, which have a standard diameter of 1 mm, arexe2x80x94in addition to their comparatively low manufacturing costsxe2x80x94their ease of use in the making of connections (connectors, splices) and in the construction of branching and coupling elements, as well as in their ease of end-surface treatment and their low sensitivity to external mechanical, physical and environmental influences. The transmission wavelengths presently usable lie in the visible spectral range.
The following remarks relate to the example of polymeric optical fibers made of PMMA, but apply analogously to other polymeric optical waveguides, as well.
Principal disadvantages of polymeric optical fibers made of PMMA lie in the material-inherent high attenuation of approximately 150 dB/km at the frequently used transmission wavelength of 650 nm, and also in the fact that the sole type of fiber commercially available at present exhibits very great modal dispersion as a consequence of its exclusively step-index profile. The step-index profile of a polymeric optical fiber exhibits a constant refractive-index profile across virtually the entire fiber diameter. Consequently, the possible transmission rate is relatively low and is also significantly dependent on the coupling conditions. A transmission distance of approximately 100 m can be regarded as realizable in practice for the data rate of, for example, 125 Mbit/s which is of interest for communications. The bandwidth/length product is calculated at 1.5 MHzxc2x7km for approximately 25 m fiber length and is measured at between 5 MHzxc2x7km and 6 MHzxc2x7km.
At present, polymeric optical fibers made of PMMA are employed especially in the field of machine control at low transmission rates, as well as in the field of sensor technology, robotronics and for simple illumination systems. The use of the more temperature-stable polycarbonate fiber is under discussion as a further application in the automotive field. Furthermore, there is considerable interest in using polymeric optical fibers over short distances, i.e., over the so-called xe2x80x9clast 100 yardsxe2x80x9d, xe2x80x9cin-housexe2x80x9d or in the field of office communications, such as local area network (LAN) applications.
In order to improve the transmission characteristics and thus to widen the area of possible applications for polymeric optical fibers, intensive efforts are being made to produce PMMA fibers which have a fiber core with a graded-index profile. There has been a known attempt to construct the desired parabolic refractive-index profile through so-called xe2x80x9cinterfacial-gel copolymerizationxe2x80x9d. The different diffusion rates of various monomeric PMMA derivatives are exploited in order to produce a preform. PMMA fibers thus produced with a parabolic refractive-index profile and a core diameter of 600 xcexcm are not yet commercially available. However, they are characterized by significantly increased data-transmission rates and by good attenuation values.
However, the processing speed is very slow owing to the rate of diffusion. Since the substances to be diffused always penetrate from outside, i.e., from the cladding surface of the fiber, there are significant restrictions with regard to the form of the refractive-index profile.
On the other hand, it is known (see: W. F. X. Frank et al.: Optical Waveguides in Polymer Materials by Ion Implantation, SPIE Vol. 1559 (1991) 344-353) that the refractive index of polymers can be changed with the aid of ionizing radiation.
Proceeding from the problems described above, an object of the present invention is to provide a method which makes it possible in simple manner to produce a polymeric optical fiber having a defined refractive-index profile that permits a high processing speed and high flexibility in the design of the refractive-index profile.
The present invention therefore provides a method for making a polymeric optical fiber having a graded-index profile, the method including exposing at least one of the polymeric optical fiber, a preform of the polymeric optical fiber, and a fiber-core preform of the polymeric optical fiber to ionizing radiation of predetermined dose and wavelength while moving the respective polymeric optical fiber, the preform of the polymeric optical fiber, and/or the fiber-core preform of the polymeric optical fiber along a center line of the respective polymeric optical fiber, the preform of the polymeric optical fiber, and/or the fiber-core preform of the polymeric optical fiber at a given speed; defining a spatial distribution of an intensity of the ionizing radiation in the respective polymeric optical fiber, the preform of the polymeric optical fiber, and/or the fiber-core preform of the polymeric optical fiber over an entire irradiated length of the respective polymeric optical fiber, the preform of the polymeric optical fiber, and/or the fiber-core preform of the polymeric optical fiber using a system of optical elements so that the ionizing radiation is rotationally symmetrical; and then tempering the respective polymeric optical fiber, the preform of the polymeric optical fiber, and/or the fiber-core preform of the polymeric optical fiber for a predetermined time at a predetermined temperature so as to change a respective refractive index of respective irradiated regions of the respective polymeric optical fiber, the preform of the polymeric optical fiber, and/or the fiber-core preform of the polymeric optical fiber so as to form the graded index profile, the graded-index profile being a uniform radial graded-index profile.
In the method according to the present invention, ionizing radiation is used to cause chemico-physical changes in the material of a polymeric optical fiber which change the refractive index in the polymeric optical fiber in such a manner that a uniform, radial, graded-index profile perpendicular to the longitudinal axis of the polymeric optical fiber is formed along the entire fiber length, preferably in the core region. Furthermore, the method according to the present invention is suitable for additionally producing specific changes, if required, in the refractive-index profile of the fiber cladding. According to the present invention, a fiber preform may also be irradiated instead of the polymeric optical fiber. Likewise, it is possible for the later fiber core in the form of a preform (hereinafter referred to as xe2x80x9cfiber-core preformxe2x80x9d for reasons of simplification) to be irradiated on its own if a so-called xe2x80x9crod-in-tubexe2x80x9d process is selected for fiber production. In this case, after irradiation and, where appropriate, after tempering, the fiber-core preform is introduced with precise fit into an equally long hollow cylinder, provided as the fiber cladding material, and the fiber is drawn off jointly from said preform arrangement at higher temperature.
For the changes in the core region, the polymeric optical fiber, the preform or the fiber-core preform is irradiated at least once with the ionizing radiation while being moved in the longitudinal direction. In so doing, the irradiation should be rotationally symmetrical, so that a uniform, radial graded-index profile is actually obtained.
Given a suitable type and duration of the ionizing radiation, the change in refractive index caused by the irradiation produces the correct refractive-index profile. Tempering (holding the polymeric optical fiber, the preform or the fiber-core preform at a defined temperature, higher than the operating temperature, for a given period of time) finally optimizes the refractive-index profile in such a manner that the desired uniform, radial graded-index profile is formed.
Rotationally symmetrical irradiation may be achieved by either the polymeric optical fiber, the preform or the fiber-core preform executing a rotating motion by a predetermined angle about the fiber center line, or by the system of optical elements rotating, together with one or more radiation sources, by a predetermined angle about the fiber center line.
Electromagnetic radiation (such as UV light, X-ray or gamma radiation, etc.) may be used as ionizing radiation.
Particle radiation (such as electrons, positrons, heavy ions, etc.) may be used as ionizing radiation.
A desired spatial distribution of the ionizing radiation in the fiber may be determined by a system of optical elements. These optical elements can be reflecting surfaces, lenses, cylindrical lenses or apertures.
In embodiments of the present invention, the type, rate and course of change in refractive index as well as attenuation may be influenced by various additions to the starting material of the fiber. Likewise, additions may be used to influence the wavelength or the effect of the particle type of the ionizing radiation required for the change in refractive index. Thus, for example, by using certain additives, it is possible to ensure that visible light causes a change in the refractive index. Furthermore, specific additives can be used to incorporate active and passive control elements into the fiber.